I was just going through some stuff with some jr. devs about how structs work, and we were fooling around with what can be done with them as an exercise, and then… well. We struct (lol) something I wasn’t able to explain away. Assume:

defmodule Cat do
  defstruct type: nil, weight: nil
end 

Obviously, %Cat{variance: "tabby"} fails with keyerror, because variance doesn’t exist on the struct, and if you did go %Cat{}, you’d receive %Cat{type: nil, weight: nil}… And further if you try to go with

%{%Cat{} | variance: "tabby"}

again, it checks that you can’t add something that was defined. You get some semblence of safety when it comes to the values of what you can expect on a struct. On defined Cat struct, it’d contain the values of type and `weight.

But… Map.delete(%Cat{type: "Maine Coon"}, :weight) gives you the appearance of a valid(?) struct still.

%{__struct__: Cat, type: "Maine Coon"}

At no point does it give the indication that it’s out of step with the struct and I haven’t been able to find out how to validate that. I’d expect on create time we’d handle the checks, but when modifying post-create, there’s no integrity checks here. I thought that maybe Kernel.struct/2 might be a use to validate here, but it comes out clean.

Obviously this is a much smaller issue when you’re using Ecto and changesets, but it was something that really quirked me a bit! TIA for the explaination.